title: Feedback field control improves linewidths in in vivo magnetic resonance spectroscopy.
abstract: OBJECTIVE
  Magnetic resonance spectroscopy (MRS) experiments rely on a homogeneous and stable magnetic field within the sample. Field homogeneity is typically optimized by static B0 shimming while reproducible effects from dynamic field variation are commonly diminished by means of gradient system calibration as well as calibration based on non-water suppressed reference data. However, residual encoding deficiencies from incomplete calibration and nonreproducible field perturbations deteriorate the quality of the obtained data. To overcome this problem, we propose to adapt higher-order feedback field control based on NMR field probes for its application in MRS.
  
  
  METHODS
  To allow for field measurements simultaneously with the spectroscopy readout, radiofrequency-shielded field probes were employed. The setup was evaluated in vitro and tested in vivo for single-voxel MRS at 7T to correct for field perturbations that occur due to subject breathing and limb motion.
  
  
  RESULTS
  The in vitro experiments showed an effective field control during the MRS sequence. The resulting spectroscopy data were free of spurious signal and the achieved field stabilization improved the spectral resolution in vitro and in vivo.
  
  
  CONCLUSIONS
  High-field MRS is limited by nonreproducible field perturbations for which spatiotemporal field feedback provides a solution without compromising sequence timing and efficiency.
authors.affiliation: Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Switzerland.
authors.forename: Bertram J
authors.initials: BJ
authors.lastname: Wilm
authors.forename: Yolanda
authors.initials: Y
authors.lastname: Duerst
authors.forename: Benjamin E
authors.initials: BE
authors.lastname: Dietrich
authors.forename: Michael
authors.initials: M
authors.lastname: Wyss
authors.forename: S Johanna
authors.initials: SJ
authors.lastname: Vannesjo
authors.forename: Thomas
authors.initials: T
authors.lastname: Schmid
authors.forename: David O
authors.initials: DO
authors.lastname: Brunner
authors.forename: Christoph
authors.initials: C
authors.lastname: Barmet
authors.forename: Klaas P
authors.initials: KP
authors.lastname: Pruessmann
mesh_terms: D000465:Algorithms; D016477:Artifacts; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005246:Feedback; D066244:Proton Magnetic Resonance Spectroscopy / Q000295:instrumentation* / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation*
keywords: 7T; MRS; NMR field probes; dynamic shimming; feedback; field monitoring; real-time shimming